Posts Tagged parts

Photos of mechanical, front panel design

Portable electronic instruments. These photos show knobs, nuts, knurled, and know-how (Pete’s). These are working photos of our differential amplifier project. They show how we can get the electrical boards to line up with the mechanical connectors, and controls. And how an extra layer of plexi can be used to add a nice finish, and provide a solution to the incompatible height of the controls (note the nut is recessed by the plexi).

overall view of assembly, showing how it slides into the case

overall view of assembly, showing how it slides into the case

close up of the BNC connectors

close up of the BNC connectors

BNC aignment of pcb, fron panel, and mounting BNC panel

BNC aignment of pcb, fron panel, and mounting BNC panel

cardboard pcb mock-up, with the real board and assembly behind it

cardboard pcb mock-up, with the real board and assembly behind it - we like to make quick mock ups as we design, just to check everything is going well

Detail of the plexi layer used to 'bury' the nut needed to hold the front panel control. The plexi is an additional part, but the incompatible heights of the controls meant there had to be some way to accomodate all these part variations.

Detail of the plexi layer used to 'bury' the nut needed to hold the front panel control. The plexi is an additional part, but the incompatible heights of the controls meant there had to be some way to accomodate all these part variations.

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Precision Resistors, low tempco

Here’s a couple sources for precision resistors.

One source seems to provide a great price/performance trade, the other is really the best quality resistor you’ll be able to buy. BUT precision low drift resistor arrays require careful inspection of the data sheet. Here’s a couple examples.

Recently I noticed that Maxim IC, (famous makers of 5V powered RS232 interface chips), have begin selling a pair of precise resistors in a SOT23 package, at an attractive price.

This family of parts, the MAX5491, claims to have 2ppm/degC resistor to resistor drift of the pair in the same package. Take care – sometimes the large print giveth, and the small print taketh away – the absolute tempco is spec’d as 35ppm/degC, so if you compare two different individual SOT23 units with each other, they may drift more that that nice small 2ppm value at the top of the spec sheet!

link to the Maxim-IC precision resistor array MAX5491 data sheet

For better matching, the VFCD1505 parts made by Vishay, also available at DigiKey, are spec’d for 0.2ppm/degC drift, or 10X better than the Maxim parts (and, well, about 5.7X the price, at $20/qnty 1, vs the $3.50 qnty 1 at DigiKey).

link to the Vishay VFCD1505 precision resistor data sheets

I like that Vishay also details subtle values such as ‘voltage coefficient’ and the current noise (or sometimes called ‘excess noise’, noise that is beyond the thermal noise of the resistor value).

The specifications of the Vishay parts are about where the state of the art is for actual resistors you can buy. Better resistors can be found only at NIST.

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Custom front panels

Here’s a vendor we recommend, and a photo of some parts they made for us.

photo of parts made by Front Panel Express for our Diff Amp project

When you make one or 10 of something it can be difficult to make it look ‘real’, that is, to make it look finished. Sometimes the Dymo-Marker labels are OK, say in a sci-fi movie, for your Custom Flux-Capacitor. But other times, you want the prototype to look clean.

We have enjoyed using the services of FrontPanelExpress to make some custom front panels for our projects. They provide free software, that’s simple and easy to use, and you upload the files to their on line ordering … and you get great parts back. A variety of anodized aluminum options and thicknesses.

http://www.frontpanelexpress.com/

They will engrave text, add paint colors into the text -makes a very nice professional looking prototype or short run of parts.

Just imagine – they can easily make a D-shaped hole for the BNC – so it won’t UNSCREW and FALL OUT !!! That alone is worth the price of admission. I can’t find my D-shaped drill bit …

They can do a lot of things that are a pain to do by hand – countersink holes, nice RS-232 type D-cutouts, square holes, etc. Check out their site for some examples.

We also made a Plexiglass panel – this allows us to compensate for the very annoying differences in height of the switches and knobs – and it allows simple paper graphics to be protected. We also considered making a glowing logo, but have not yet done that.

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attenuators, connectors, etc.

Here’s where to get quality attenuators and connectors

Testing any high gain low noise amplifier requires a nice clean attenuator.

You need to drop the level of your function generator,

or that x1000 gain amp would need to supply 100 V output.


Pasternack Enterprisese sells a nice 30dB atten for about $42.

Their part number is PE7000-30.


If you put 2 of these in series, you have about x1000 attenuation.

(These assume a 50 Ohm load, so buy one of those too).

Here’s a link to their website:

http://www.pasternack.com/

Pasternak weblink

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